1. Field of the Invention
The present invention relates to a flash memory structure and its method of manufacture. More particularly, the present invention relates to a vertical type of flash memory structure.
2. Description of the Related Art
Flash memory generally consists of a floating gate for storing electric charges and a control gate for controlling data transfer. The floating gate is inserted between the control gate and a substrate. The floating gate is in a `floating` state because the gate is not connected any external circuits. The control gate is connected to a word line while the drain terminal of each flash memory cell is connected to a bit line.
In a conventional flash memory, neighboring flash memory cells are electrically isolated from each other by field oxide (FOX). However, field oxide tends to occupy a lot of chip area. In addition, the source/drain terminals and the floating gate of each flash memory cell also occupy a lot of chip area. If area layout of source/drain terminals and floating gates of a flash memory are reduced without changing the devices'structures, performance of the memory device will be affected.
Although shallow trench isolation can replace conventional field oxide as an electrical insulator between neighboring flash memory cells so that the level of integration is increased, area occupation of STI is still limited by photoresist resolution.